The present invention relates to a roll press or roll pressing mill with two opposed pressing rolls having a roll gap between them, a material charging device projecting into the roll gap, and means for removing gases from the roll gap.
In the case both of materials for pressing which have a high air content, for example fine-grained materials which have fluid like properties due to the natural inclusion of air, and also of materials which are crushed in the material bed of a roll press and thereby converted into solid agglomerates, difficulties frequently arise with regard to the deaeration of the material. Air which is enclosed by the material and enters the roll gap is highly compressed together with the material to be pressed. This can result in the removal of stress after the exit from the roll gap in the agglomerates produced by the roll press in the form of scabs or briquets being destroyed. Enclosed amounts of air can also result in scab or flake formation, or can make it impossible to form satisfactory briquets, or to achieve the necessary pressures for burden comminution.
Adequate deaeration of the material to be pressed represents a solution to these problems. Proposals are known for this.
In the case of a roll press of the kind having two opposed pressing rolls having a roll gap between them, a material charging device projecting into the roll gap, and means for removing gases from the roll gap (as in U.S. Pat. No. 3,029,723), there is arranged in the charging hopper vertically above the roll gap a compartment which projects with its lower end, which is constructed permeable to air, into the roll gap and which is connected through the front-side walls of the feed hopper to air outlet lines. Such deaeration devices arranged inside the charging hopper have the disadvantage that they impede the flow of material. The very small air inlet openings required in this case at the lower end of the compartment, of the order of magnitude of 1/10 mm and smaller, moreover easily lead to blockages of the openings, which make the device ineffective.
There is also known in the case of roll presses with screw conveyors to provide on the charging hoppers in which the screws rotate of openings or spaces which are covered with filter media and to which a suction fan is connected. In the case of a roll press according to U.S. Pat. No. 3,114,930, a space, to which the suction fan is connected, is to be separated off above the screw by a permeable wall extending obliquely to the axis of the charging screw. In a further known arrangement according to U.S. Pat. No. 3,269,611, suction openings covered with filter media are provided in the conical charging hopper wall above the screw. In this case the surfaces of the filter media are to be kept open by skimmers connected to the screw shaft.
It is further known in German OLS 37 33 500 to provide venting (deaerating) outlets in the wall elements surrounding the roll gap, which outlets are covered with filters to prevent the escape of fine-grained material to be compressed.
Also known from German ALS 17 57 093 is a roll mill in which the material to be comminuted is fed in a continuous stream which is accelerated in a gravity drop shaft which ends at a distance from the roll gap, up to about the peripheral speed of the rolls. Entrained air in this case escapes above the rolls and is returned through a duct provided with louvers into the gravity shaft in order to prevent the material in the gravity shaft from being decelerated by air resistance. Air introduced with the material flow through openings above the roll gap is drawn off to prevent extraction of dust.
All these known devices have the common disadvantage that, together with the air to be carried away, particularly the fine fractions of the material for pressing are necessarily entrained by the escaping or exhausted air. These fine fractions of the materials lead however to blockages on both sides of the filter medium which are particularly difficult to remove.